<SELECT ID="myselect" onchange="alert('' + this.selectedIndex); if(this.selectedIndex == 3) { this.multiple = !this.multiple; } return false;" multiple>
  <option ID="option0">0
  <option ID="option1">1
  <option ID="option2">2
  <option>3
</SELECT>
<script>
   iSelect = document.getElementById('myselect');
   iOption1 = document.getElementById('option1');
   iOption2 = document.getElementById('option2');
</script>
<input type="submit" value="Change1" onclick="iSelect.selectedIndex = 1; alert('option1=' + iOption1.selected);">
<input type="submit" value="Change2" onclick="iSelect.selectedIndex = 2; alert('option2=' + iOption2.selected);">
<input type="submit" value="Change3" onclick="iOption1.selected = true; alert('selectedIndex=' + iSelect.selectedIndex);">
